Materials are often produced by heating them to a liquid state and then allowing the molten material to cool. The way in which a molten material cools to a solid state can impacts the properties of the end material and controlled cooling or “quenching” can be exploited to tailor the material properties by adjusting the microstructural make-up. In the solid state, many materials form coherently diffracting domains, which are also known as grains or crystallites. When the material is in powder form and the average grain size is in the range of 100 nanometers to 10 micrometers, the powders may be referred to as micropowders. When the average grain size is equal to or less than 100 nanometers in all dimensions, the powder may be referred to as a nanopowder. When mean grain sizes of respectively, 100 nanometers to 10 micrometers, or equal to or less than 100 nanometers, are present in a bulk material, that material is said to be “microstructured” or “nanostructured”. It shall be understood that the dimensions for micropowders, nanopowders, microstructured, or nanostructured discussed above are merely illustrative and nonlimiting. It shall be understood that these definitions do not strictly adhere to the ranges discussed.
To understand the scale of size reduction, it is useful to consider a single crystal of common table salt, or sodium chloride. The common table salt form is a cube of approximate dimensions of 300 micrometers on a side. Compared to this salt crystal, a micropowder particle that is cubic with a dimension of 300 nanometers is a thousand times smaller in dimension on each side and has one billionth of the volume. The surface area to volume ratio of a cube is inversely proportional to length, so if one billion cubes of dimension 300 nanometers on a side were arranged to make up a cube the size of the example table salt crystal, there would be a total surface area that was one thousand times as great, due to all of the surfaces at the interfaces between cubes. If one considers a particle that is cubic with dimension of 30 nanometers on a side, then there is another factor of 1000 reduction in volume and a factor of ten increase in surface area to volume ratio. The concept of surface area of crystalline grains is an important one when considering nanostructured bulk materials since interfaces occur at grain boundaries at the surfaces of grains. With smaller grains, a given volume of bulk material will have more interfaces.
A material with small grains can have very different macroscale properties compared to a more conventional, large grained bulk solid even though both have an identical chemical make-up. For example, as the grains of a densified bulk material are reduced in size (depending upon material) from tens of micrometers to a few micrometers and further into the nanoscale, the yield strength increases—a phenomena characterized by the Hall-Petch relation and given by the formula:
                                          σ            y                    =                                    σ              0                        +                                          K                y                                            d                                                    ,                            (        1        )            where σy is the yield strength, d is the grain size and σ0 and ky are material constants. The Hall-Petch relation does not hold as grain sizes are reduced into the sub 100 nm region because the abundance of weaker grain boundaries softens the structure, allowing a greater degree of plastic deformation and ductility. Material strength may continue to increase with grain size reduction but that increase occurs by an amount that is less than the level suggested by equation (1). As such, a nanostructured bulk material may exhibit both higher strength and higher ductility (together a property known as “toughness”) than its larger grained counterpart.
Another example of the use of small grain sizes to engineer desirable material properties is its use in reducing thermal conductivity in thermoelectric materials. In a nanostructured bulk material, the presence of a high density of grain boundaries, lattice defects and scattering centers can serve to decrease the thermal conductivity of a material by impeding phonon transport. Phonons are quanta of lattice vibration and they have a distribution of characteristic wavelengths that are material dependent. In a single large crystal, the mean free path of the phonons can be many wavelengths long. In contrast, the presence of many grain boundaries in a nanostructured material shortens the phonon mean free path and thereby reduces the thermal conductivity due to phonon scattering at grain boundaries. For example, reduced thermal conductivity is an attractive feature for a thermoelectric heat pump because it reduces the lattice heat flow that is counter to the desired pumping direction. Reduced thermal conductivity is attractive for a thermoelectric generator because it reduces the amount of diffusive heat energy flux that passes through the thermoelectric elements without being converted to electricity.
Fine grained bulk materials may be made by consolidating powders having a large proportion of micro-scale or nano-scale crystallites. These fine grained powders can be produced through a variety of well-documented processes including mechanical milling, chemical synthesis, melt-spinning and gas atomization. However, it is challenging to fashion a dense bulk material from powders without a significant increase in the mean size of the grains.
Converting a powder into a solid may be accomplished through a combination of compaction and heat treating. The objective of the compaction step is to obtain high density. The heat treating step then serves to enhance interparticle bonding and reduce intergranular voids. Compaction and heating can be carried out simultaneously.
Compaction can be accomplished through one of a number of approaches. In uniaxial die compaction, a punch compresses powder in a rigid-walled die. Isostatic pressing techniques use a flexible die, which is sealed with powder inside and is submerged in a fluid chamber which is then hydrostatically compressed. In contrast to uniaxial and isostatic compression, which are static compression techniques, shockwave consolidation represents a means to accomplish compaction dynamically. In this technique, an explosive shockwave travels down a powder filled tube, with the very high energy compaction wave causing powder particles to plastically deform and consolidate. During this process there are two sources of heating. First, the surface energy of the powder is higher than the interface energy of the compact. The extra energy gets converted to heat. Second, the deformation of the individual particles and rearranging of atoms on the interface cause heating due to internal friction. More heating occurs at the surface of individual particles, in some cases causing melting at the interfaces, which are then cooled by the particle. As the shock wave travels through the powder, it has to supply the energy for the plastic deformation of the individual particles. This effect serves to diminish the intensity of the wave as it travels from the outside of the pipe to the center. At the same time, the shockwave converges from all radial directions towards the center of the pipe. This convergence serves to increase the intensity of the wave as it travels from the outside of the pipe to the center. These two effects should be carefully balanced to obtain a uniform consolidate.
Shockwave consolidation sometimes yields an incompletely bonded and/or low density material. A post heat treatment then becomes necessary, but the applied temperatures can cause undesirable grain growth. When a fine-grained end product is desired, the challenge is obtaining high density and good interparticle bonding while preserving small grains. The key variables of temperature, pressure and time are all important and can be traded off to obtain a given result.
Various shockwave consolidation techniques have been disclosed, such as in “Shock-Wave Consolidation of Rapidly Solidified Superalloy Powders”, by M. Meyers, B. Gupta and L. Murr, Journal of Metals, vol. 33, no. 10, October 1981, pp 21-26, U.S. Pat. No. 5,826,160 to Kecskes, U.S. Pat. No. 7,364,628 B2 to Kakimoto et al., and U.S. Pat. No. 8,668,866 to Rubio and Nemir. However, it can still be challenging to obtaining high density and good interparticle bonding while preserving small grains in the bulk material with known shockwave consolidation processes.
Systems and methods for producing a dense, well-bonded, fine grained bulk material are discussed further herein. The systems and methods may utilize shock-wave consolidation or any other suitable consolidation technique as a mechanism for compacting powders into a bulk solid while preserving grain size. The resulting bulk material within an enclosing container may also be thermally processed to enhance densification and inter-particle bonding.